1. Field of the Invention
The present invention relates to a radiation imaging apparatus including a radiation detector.
2. Description of the Related Art
An X-ray imaging apparatus using an X-ray as a type of radiation has been widely used, for example, in the fields of industrial nondestructive inspection or medical diagnosis. In a conventional X-ray imaging apparatus, a subject to be imaged is placed between an X-ray generation unit and an imaging unit having a built-in X-ray detector. In other words, the X-ray generation unit and the imaging unit face each other on opposite sides of the subject. The X-ray generation unit irradiates the subject with controlled amounts of X-ray radiation, and the imaging unit detects the X-ray radiation which is transmitted through the subject and received by the X-ray detector to obtain image information (an image signal).
In recent years, with the development of digital technology, digital X-ray detectors using various image sensors which convert the detected X-ray to an electrical signal have been remarkably advanced. The X-ray imaging apparatus having the digital X-ray detector is currently employed in many facilities instead of an imaging apparatus which performs analog imaging by a conventional film/screen technique using a phosphor and a photosensitive film.
A flat detector (a flat panel) as a representative X-ray detector which processes a digital signal is generally configured with a combination of a phosphor which converts the X-ray to visible light, a photoelectric conversion element, and a photoelectric conversion panel in which pixels having switching elements are disposed in a two-dimensional matrix array form. The X-ray irradiated to the X-ray detector is converted to visible light by the phosphor and then to electrical charges by each pixel. The electric charges (the electrical signal) are read from each pixel and are output as the image signal.
In recent years, with the spread of the digital X-ray detector, requirements for technical problems such as miniaturization of the X-ray imaging apparatus, image quality improvement of the captured image, stability of the image have become all the more severe. As a consideration point for satisfying the requirements, there is heat generation of electrical components which are used inside the apparatus and dispensable for digitalization, and a heat dissipation unit which efficiently dissipates heat is required. Heat dissipation is required not only to assure a normal operation and durability of the electrical components, but also to prevent or minimize an inside temperature rise of the X-ray imaging apparatus from changing characteristics of the X-ray detector.
Heat dissipation techniques for cooling an electronic device contained within a housing structure are known. For example, a known heat dissipation technique of a forced ventilation air-cooling type introduces external air while discharging high-temperature air from inside the housing to cool a device disposed inside the housing. According to this technique, it is difficult to prevent foreign substances from entering inside the housing during operation of the device because a ventilation hole is provided where a cooling unit resides. Since a ventilation hole is typically formed in a part of an exterior member thereof for ventilation, light and moisture can easily enter from the outside.
In an X-ray imaging apparatus, it is highly preferable that the X-ray detector detects only light converted from the X-ray by the phosphor, so as to output the image signal with low noise. Therefore, in the X-ray imaging apparatus, it is undesirable that unnecessary light (e.g., ambient light) does not enter into the neighborhood of the X-ray detector. Generally, the housing plays an important role of shielding electromagnetic noise through the exterior member. However, when an opening such as the ventilation hole is formed in the housing, a route through which the electromagnetic noise enters is formed. Therefore, driving of internal electric circuits may be disturbed. For these reasons, it is undesirable that the opening is formed in the exterior member of the X-ray imaging apparatus which is operating.
In an attempt to address the above-described issues, Japanese Patent Application Laid-Open No. 10-177224 proposes a technique in which a shutter for opening and closing the ventilation hole is disposed on the inside of the ventilation hole, and a fan is disposed on the inside thereof. It is sufficiently effective to use the technique in an X-ray imaging apparatus for taking a still image in which in a driving period of the X-ray imaging apparatus, a time taken for actual imaging operation is very short. However, when external air is introduced into the inside of the X-ray imaging apparatus, moisture or dust is also inevitably introduced at the same time. The moisture or dust introduced into the inside accelerates deterioration of the phosphor or semiconductor devices of the X-ray detector. Further, the accumulated dust may become an obstacle to heat dissipation of the electrical components.
As one of countermeasures for solving the dust issue, an air filter made of a porous synthetic resin-based material may be disposed in the ventilation hole for introducing external air. However, it is ineffective in solving the moisture issue, and since a time and effort for maintenance for cleaning or replacing the air filter are required, it is inconvenient.
An X-ray imaging apparatus as illustrated in FIG. 12 has been disclosed to address the above-discussed issues.
FIG. 12 is a view illustrating an example of a schematic configuration of an imaging unit of a conventional X-ray imaging apparatus. Particularly, in an imaging unit 1200, disposed are a housing (a box) which includes an X-ray detector 1201 and electrical components 1204 thereinside and is almost hermetically sealed and a fan motor 1213 for ventilation which is disposed inside an exterior member which is the outside of the housing as an appearance member.
An X-ray irradiated from an X-ray generation unit (not illustrated) is incident on the X-ray detector 1201 in a direction indicated by the X-RAY arrow. The X-ray detector 1201 converts the incident X-ray to an electrical signal (an image signal). The image signal is input to a signal processing substrate 1203 from the X-ray detector 1201 via a flexible cable 1202. On the signal processing substrate 1203, plural electrical components 1204 are mounted. The electrical components 1204 are configured to process the image signal input from the X-ray detector 1201 and to the X-ray detector 1201.
Accordingly, the electrical components 1204 serve as the main heat generation sources in the imaging unit 1200. As a heat dissipation unit for preventing generated heat from being concentrated, a thermal conduction sheet 1205, a thermal conduction member 1206, and a heat dissipation frame 1207 are disposed to configure a heat transfer route.
The thermal conduction sheet 1205 is composed of, for example, an elastic body sheet made of silicone rubber that transmits the heat of the electrical component 1204 to the thermal conduction member 1206. The thermal conduction member 1206 is made of, for example, a material with high thermal conductivity such as copper or aluminum that efficiently transmits the heat to the heat dissipation frame 1207. Further, when the amount of heat generation is large, as the thermal conduction member 1206, a special thermal conduction tool such as a heat pipe or a component using a graphite material may be used. The heat dissipation frame 1207 is a member that spreads heat transferred from the thermal conduction member 1206 and transmits heat by the convective flow of air or heat radiation, and discharges the heat. The thermal conduction member 1206 also forms part of a frame of the housing structure.
A detector frame 1208 includes the X-ray detector 1201 and the signal processing substrate 1203 therein and holds the X-ray detector 1201 and the signal processing substrate 1203 via members which are not illustrated. The detector frame 1208 is made of metal with electrical conductivity such as iron, stainless steel or aluminum. The detector frame 1208 also serves to protect the X-ray detector 1201 and the electrical components 1204 which are included therein from external electrical noise and prevents internal electromagnetic waves from being emitted to the surrounding environment.
An X-ray transmitting portion cover 1209 transmits the X-ray so that the X-ray cannot deteriorate and protects the X-ray detector 1201. As the X-ray transmitting portion cover 1209, a carbon sheet made of a material with high X-ray transmittance or a thin aluminum sheet is used. The heat dissipation frame 1207, the detector frame 1208, and the X-ray transmitting portion cover 1209 are combined to form the housing structure and are configured to almost hermetically seal a space including the X-ray detector 1201, the signal processing substrate 1203, and the electrical components 1204. Therefore, external light does not reach a photosensitive portion of the X-ray detector 1201, and moisture and dust are prevented from entering into the neighborhood of the X-ray detector 1201 and the signal processing substrate 1203. Further, an effect of shielding from intrusion of the electrical noise and emission of the electromagnetic wave can be increased.
An exterior cover 1210 is disposed on the outside of the detector frame 1208 as a member which constitutes an appearance of the imaging unit 1200. The exterior cover 1210 is coupled to the detector frame 1208 via a portion which is not illustrated to support the detector frame 1208. The exterior cover 1210 is supported from the outside via a mechanism which is not illustrated. In the exterior cover 1210, ventilation holes 1211 and 1212 are formed. The fan motor 1213 is disposed at a position adjacent to the ventilation hole 1211. The fan motor 1213 performs a blowing operation for discharging air in the imaging unit 1200 through the ventilation hole 1211. The ventilation hole 1212 serves to take air outside the imaging unit 1200 into the inside.
Heat inside the housing is spread around the heat dissipation frame 1207 and transferred to contacting air from a surface of the outside of the housing. Accordingly, the air flows in a direction illustrated by an arrow B, moved by the fan motor 1213, and is discharged to the outside of the imaging unit 1200 through the ventilation hole 1211.
Since the ventilation holes 1211 and 1212 are formed in the exterior cover 1210, the external light, moisture, or dust may be introduced into the imaging unit 1200. However, it is configured to prevent the external light, moisture, or dust from being introduced up to the inside of the detector frame 1208 in which the X-ray detector 1201 and the signal processing substrate 1203 are disposed.
Incidentally, in an example illustrated in FIG. 12, a configuration of ventilation air-cooling type using the fan motor 1213 is illustrated, but for low power consumption, the X-ray imaging apparatus may have a configuration having no fan motor 1213, that is, a configuration in which heat dissipation is achieved by natural air-cooling.
However, the conventional X-ray imaging apparatus has problems described below.
With the development of a recent signal processing technique and a signal transmission technique, an environment for processing the image signal at a high frame rate has been made. The digital X-ray detector has been used in the X-ray imaging apparatus which takes only a still image until now, but an opportunity of using an imaging apparatus which takes a moving image in which an image is continuously taken for a long time or a computed tomography (CT) in which three-dimensional image information is processed is recently gradually increased.
When a moving image is taken at a high rate, a driving frequency of the electrical component per unit time is increased, and power consumption is also inevitably increased. Incidentally, the amount of heat generation is also increased. Heat generation of the X-ray detector does not have a significant impact in low-frequency driving for taking a still image. However, when continuous driving for taking a moving image is performed for long periods of time, the heat generation may greatly affect a characteristic of the X-ray detector itself.
When a high-low distribution of the heat generation amount occurs in a surface of the signal processing substrate 1203 due to heat generation of the electrical components mounted on the signal processing substrate 1203, it affects the X-ray detector 1201 disposed in the same space. Therefore, temperature unevenness between a high temperature part and a low temperature part occurs in a panel surface of the X-ray detector 1201, and a detection characteristic of each part becomes different. Thus, the quality of an output image deteriorates. Furthermore, when the X-ray image apparatus starts its operation, the temperature of the X-ray detector is low directly after power is supplied, whereas the temperature becomes high by influence of heat generation while driving is continuously performed. Therefore, the detection characteristic of the X-ray detector varies according to the lapse of time, causing a stability problem.